run-tests: define a custom TestSuite that uses _executetests()
We now have a custom unittest.TestSuite implementation that uses
_executetests() and thus knows how to execute tests concurrently.
Running tests in --unittest mode will use this TestSuite.
Since the TestSuite handles concurrency, the warnings around --jobs and
--loop have been removed.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.